e2e-tests.md: explain usage of Feature tag for alpha/beta tests

While we now have the ability to support tests properly which only depend on
alpha or beta feature gates, we still haven't migrated all jobs. So for now,
developers still need to follow the old practice of tagging such tests with
both [Feature] and [FeatureGate].

Author: pohly

contributors/devel/sig-testing/e2e-tests.md

@@ -603,7 +603,13 @@ refine requirements:
     Conceptually, these are non-default requirements as defined above under
     `[Feature:.+]`, but for historic reasons and the sake of brevity they don't
     have that prefix when embedded in test names. They *do* have that prefix in the
-    Ginkgo v2 label, so use e.g. `--filter-label=Feature: containsAny Alpha`.
+    Ginkgo v2 label, so use e.g. `--filter-label=Feature: containsAny Alpha` to
+    run them. The normal `--filter-label=Feature: isEmpty` excludes them.
+
+    Note that at the moment, not all jobs filter out tests with `Alpha` or `Beta`
+    requirements like that. Therefore all tests with such a requirement also
+    have to be annotated with a `[Feature]` tag. This restriction will be lifted
+    once migration of jobs to `--filter-label` is completed.
 
 Every test should be owned by a [SIG](/sig-list.md),
 and have a corresponding `[sig-<name>]` label.